Brittle materials utilized in the production of cement conventionally are comminuted to reduce the material to fine particles. Such brittle material may include, for example, cement, clinker, gypsum and granulated blast furnace slag, which typically are comminuted either separately or as a mixture and constitute the principal components of the cement product. In this case the comminution of the material for grinding takes place in a closed circuit, in which comminution product coming from a mill is classified in a classification zone into fine material, which is drawn off as finished product, and oversize material, which is not yet sufficiently comminuted and is returned to the mill for further comminution.
It is generally known in the art that cements or similar products which are ground in a closed circuit with a high-pressure grinding roll mill differ in their quality characteristics from those products which are produced in roller mills, ball mills or the like. These differences of quality are expressed principally in a greater use of water than, for example, with ball mill products for the standard stiffness of cement mortar or concrete. Starting from this problem, therefore, it is proposed in EP-A-406 591 inter alia that the agglomerated comminution product, so-called scabs, coming from a high-pressure grinding roll mill should first of all be disagglomerated and then a part thereof should be fed directly back to the mill, whilst the other part of the comminution product is separated in two air separators which are connected in parallel and set to different degrees of fineness, i.e. the proportion of the comminution product passed to the classification zone is first of all divided in a material stream divider into two part-streams, of which one part stream is delivered to one air separator and the other part-stream is delivered to the parallel-connected second air separator. Whereas the entire quantity of oversize material from each separator is returned directly to the mill, the fines from both parallel-connected separators are mixed together and drawn off. This should make it possible for the particle size distribution the finished product to be set specifically over a large range during energy-saving comminution in order to be able to achieve a particularly favourable behaviour during processing with the desired strength development.
The object of the invention is to make further developments in such a way that with specific setting possibilities in the particle size distribution of the finished product the desired product quality can be achieved with a further reduced and relatively low specific energy consumption.